No abstract

No abstract

A characterization program has been developed at Hanford to image past leaks in and around the underground storage tank facilities. The program is based on electrical resistivity, a geophysical technique that maps the distribution of electrical properties of the subsurface. The method was shown to be immediately successful in open areas devoid of underground metallic infrastructure, due to the large contrast in material properties between the highly saline waste and the dry sandy host environment. The results in these areas, confirmed by a limited number of boreholes, demonstrate a tendency for the lateral extent of the underground waste plume to remain within the approximate footprint of the disposal facility. In infrastructure-rich areas, such as tank farms, the conventional application of electrical resistivity using small point-source surface electrodes initially presented a challenge for the resistivity method. The method was then adapted to directly use the buried infrastructure, specifically the steel-cased wells that surround the tanks, as "long" electrodes for both transmission of electrical current and measurements of voltage. Overcoming the drawbacks of the long electrode method has been the focus of our work over the past 7 years. The drawbacks include low vertical resolution and limited lateral coverage. The lateral coverage issue has been improved by supplementing the long electrodes with surface electrodes in areas devoid of infrastructure. The vertical resolution has been increased by developing borehole electrode arrays that can fit within the small-diameter drive casing of a direct push rig. The evolution of the program has led to some exceptional advances in the application of geophysical methods, including logistical deployment of the technology in hazardous areas, development of parallel processing resistivity inversion algorithms, and adapting the processing tools to accommodate electrodes of all shapes and locations. The program is accompanied by a full set of quality assurance procedures that cover the layout of sensors, measurement strategies, and software enhancements while insuring the integrity of stored data. The data have been shown to be useful in identifying previously unknown contaminant sources and defining the footprint of precipitation recharge barriers to retard the movement of existing contamination.

A laboratory study was conducted to investigate 15N cross‐contamination errors between distilled standards [1 mg N as (NH4)2SO4] with various 15N excess contents (0, 3, or 10% excess 15N). Several stills differing in rate of distillation and construction were tested. We also compared the effectiveness of the ethanol (ETOH) cleansing technique with the (NH4)2SO4 cleansing technique in reducing cross‐contamination errors. The ETOH wash was less effective than the (NH4)2SO4 wash in reducing cross‐contamination errors for samples with normal abundance 15N but was equally effective in reducing cross‐contamination errors for samples with high‐enrichment 15N. The (NH4)2SO4 wash sometimes significantly diluted high‐enrichment 15N samples.Ammonia seemed to be absorbed and sporadically released from the nonglass components of the partial‐glass still. Replacing the rubber and teflon components with glass and decreasing the glass surface area of the distillation unit decreased cross‐contamination errors and increased the percent recovery of atom % 15N from the distillation unit.

No abstract